Reproduction

Heritable cryptic chromosomal translocations in Holstein bulls detected by sperm sorting technology

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Abstract

Balanced reciprocal chromosomal translocations (rcp) occur when segments from two non-homologous chromosomes are exchanged following a double-strand break. These translocations are termed "balanced" because they do not alter the overall genomic content of the cell. Traditional cytogenetic methods fail to detect rcp when the derivative chromosomes are similar in size to normal bovine chromosomes, since detection relies on the visual inspection of karyotypes. This limitation has impeded efforts to evaluate the prevalence of rcp in cattle and their effect on bull fertility. Moreover, it remains unclear whether parents can transmit rcp to viable offspring. Here we report a case of rcp in a Holstein parent-offspring pair that was uncovered by using the proprietary flow cytometric sperm sorting technology from STgenetics®. Semen samples from both parent and offspring did not show a typical bimodal distribution of fluorescence intensity for sperm cells stained with a DNA-selective dye, as expected for normal X- and Y-bearing sperm cell populations. Instead, two additional peaks appeared at the opposite ends of the distribution, representing Y-bearing cells carrying the smallest derivative chromosome and X-bearing cells carrying the largest. These tails were isolated for whole genome sequencing at 30x coverage with Illumina paired-end short reads. By analyzing genome-wide scaled sequencing depth differences between the two tail populations, the chromosomes involved in the rcp event became readily apparent, revealing a t(1;25) translocation. Inspection of the edges of sequencing depth changes on chromosomes 1 and 25 in Integrative Genomic Viewer (IGV) resolved the breakpoints to single nucleotide precision, estimating derivative chromosome lengths of 160.3 Mbp and 40.5 Mbp - closely matching the original sizes of chromosomes 1 (158.5 Mbp) and 25 (42.3 Mbp). G-banding karyotyping performed at the Molecular Cytogenetics Laboratory of Texas A&M University returned normal results for both animals. However, fluorescence in situ hybridization (FISH) using probes for chromosomes 1 and 25 confirmed the translocation. Fertility data for the sire showed conception rates of 7.7% (1 pregnancy in 13 inseminations) with conventional and 18.3% (17 pregnancies in 93 inseminations) with sexed semen. These conception rates were significantly lower than the averages of 42.4 ± 9.3% for conventional and 43.1 ± 9.5% for sexed semen observed in 1,028 contemporary bulls with ≥10 services. This placed the rcp carrier bull below the 1st percentile for conception rates. These findings reveal that cryptic, heritable rcp can go undetected by cytogenetics analysis in Holstein cattle, contributing to and perpetuating sub-fertility.

Keywords: 2026

How to Cite:

da Silva, V., Utsunomiya, A., Cedraz, H., Canabrava Gouveia, G., Mendoza Cerna, M., Raudsepp, T., Gonzalez-Marin, C., Deeb, N. & Ross, P., (2026) “Heritable cryptic chromosomal translocations in Holstein bulls detected by sperm sorting technology”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2286824. doi: https://doi.org/10.31274/wcgalp.24134

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